This application is related to Japanese Patent Applications Nos. 2001-019585 filed on Jan. 29, 2001, and 2002-10153 filed on Jan. 18, 2002 whose priorities are claimed under 35 USC xc2xa7119, the disclosure of which is incorporated by reference in its entirety.
1. Field of the Invention
The present invention relates to a photovoltaic cell, a process for producing the same and a process for producing a solar cell using the same.
2. Description of the Related Art
A solar cell utilizing sunlight is receiving attention as an alternative energy source instead of biofuel, and various investigations have been carried out. The mainstream of solar cells that are currently subjected to practical includes a cell formed with a polycrystalline silicon substrate and a cell formed with amorphous silicon. The former is high in production cost, and a large amount of energy is consumed in the production process thereof. The later is high in production cost, and semiconductor gas materials, which involve difficulty on handling, are necessarily used in the production process thereof.
Japanese Unexamined Patent Publication Nos. Sho 58 (1983)-88169, Sho 59 (1984)-124772 and Hei 1 (1989)-289173 disclose those silicon solar cells, in which solar cells having different absorption spectra are accumulated on the same substrate. However, the solar cells cannot solve the foregoing problems.
A dye-sensitized solar cell is receiving attention as an alternative solar cell instead of the silicon solar cells.
The basic structure of the dye-sensitized solar cell includes a transparent electrode, a porous photovoltaic layer, a hole transporting layer and a counter electrode. One or two kinds of photosensitizing dyes having absorption spectra in the visible light region are adsorbed on the surface of the semiconductor of the porous semiconductor layer having a single layer structure. Upon operation of the solar cell, the porous photovoltaic layer is irradiated with light to generate electrons on the side of the porous photovoltaic layer, and the electrons migrate to the counter electrode through an electric circuit. The electrons migrating to the counter electrode are transported with ions in an electrolyte to go back into the porous photovoltaic layer. The process is repeated to take out electric energy. Japanese Patent No. 2,664,194 and WO94/05025 disclose dye-sensitized solar cells utilizing migration of light-induced electrons of a metallic complex.
However, in the case where the sensitizing dye disclosed in Japanese Patent No. 2,664,194 is used, the absorption wavelength range of the dye used for photovoltaic conversion is narrower than the spectrum of sunlight, and sunlight cannot be effectively utilized, whereby it provides a low photovoltaic efficiency in comparison to the silicon solar cells. In order to solve the problem, Japanese Unexamined Patent Publication No. 2001-76772 discloses an improvement of the short circuit electric current density (Jsc) by controlling the particle diameter of titanium oxide. However, according to J. Am. Chem. Soc., vol. 115, pp. 6382-6390 (1993), in the case where the external quantum efficiency of the dye-sensitized solar cell exceeds 80%, the internal quantum efficiency exerts a value approaching 100% taking such factors as transmission, reflection and scattering by glass into consideration. This means that there is a limit to improve the external quantum efficiency due to scattering. Consequently, there is no means to obtain a higher value of Jsc other than expansion of the photosensitivity range of the dye.
In order to expand the photosensitivity range of the dye, Japanese Unexamined Patent Publication No. 2000-243466 discloses a dye-sensitized solar cell using two dyes. In Japanese Unexamined Patent Publication No. 2000-243466, a film is formed after adsorbing a photosensitizing dye on titanium oxide fine particles, and the operation is repeated to produce the solar cell. However, because a baking step is not carried out in the production process, electroconductivity among titanium oxide particles is greatly impaired, and carriers formed by irradiation light cannot effectively reach the electrode, whereby no photovoltaic current can be taken out. In the production process of Japanese Unexamined Patent Publication No. 2000-243466, furthermore, because the semiconductor layers constituting the multi-layer structure are formed in the similar production process, light taken on the light receiving surface is uniformly scattered in the first layer and the second and subsequent layers. Therefore, the amount of light that is taken in the second and subsequent layers of the porous semiconductor layer is decreased due to scattering by the first semiconductor layer, and it is difficult that an electric current is effectively taken out from the second and subsequent semiconductor layers.
An object of the present invention is to provide a photovoltaic cell of high efficiency by applying a multi-layer structure to an oxide semiconductor layer and controlling the haze ratio of the multi-layered oxide semiconductor layer over a prescribed value so as to improve Jsc and by differentiating the particle diameters of semiconductor fine particles between the first and second layers of the multi-layered semiconductor layer in such a manner that the first layer has a smaller or a uniform particle diameter for suppressing light scattering and the second layer scatter light sufficiently for taking light in high efficiency. Objects of the invention are also to provide a dye-sensitized photovoltaic cell that can conduct photovoltaic conversion of light of a wide range of spectrum from the visible region to the near infrared light region in the sunlight spectrum and to provide a photovoltaic cell having a high efficiency by improving Jsc. These photovoltaic cells can be used as solar cells of low cost.
In order to accomplish the invention, the inventors have made intensive study and found the following. A multi-layer structure is applied to a porous semiconductor layer of a photovoltaic layer, and the haze ratio thereof is controlled over a prescribed value, so as to improve Jsc. In the case where the porous semiconductor layer has a two-layer structure, the particle diameters in the first and second layers are differentiated from each other. That is, light scattering is suppressed in the first layer by using semiconductor fine particles having a small particle diameter or a uniform particle diameter, whereas semiconductor fine particles exerting light scattering are used in the second layer, so as to take the light in the second layer in high efficiency to provide a photovoltaic cell of high efficiency. The term xe2x80x9chaze ratioxe2x80x9d used herein means a xe2x80x9chaze value (%)xe2x80x9d as a standard for light transmittance, which is defined in JIS K6714.
The invention provides a dye-sensitized photovoltaic cell containing an electroconductive support, a porous photovoltaic layer constituted with a porous semiconductor layer containing a photosensitizing dye, a hole transporting layer, and a support on a counter electrode side, the porous photovoltaic layer having a multi-layer structure, and the semiconductor layer having a haze ratio at a wavelength in a visible light region of 60% or more.
The invention also provides a dye-sensitized photovoltaic cell containing an electroconductive support, a porous photovoltaic layer constituted with a porous semiconductor layer containing a photosensitizing dye, a hole transporting layer, and a support on a counter electrode side, the porous photovoltaic layer having a multi-layer structure, a process for producing the same, and a solar cell using the same.
The invention further provides a dye-sensitized photovoltaic cell, in which respective layers of the porous photovoltaic layer having a multi-layer structure are arranged in an order, from a light receiving surface, from a layer having a maximum sensitivity wavelength region on a short wavelength side in the absorption spectrum to a layer having a maximum sensitivity wavelength region on a long wavelength side in the absorption spectrum.
The invention still further provides a dye-sensitized photovoltaic cell, in which particles constituting the respective layers of the porous photovoltaic layer having a multi-layer structure have different particle diameters.
These and other objects of the present application will become more readily apparent from the detailed description given hereinafter. However, it should be understood that the detailed description and specific examples, while indicating preferred embodiments of the invention, are given by way of illustration only, since various changes and modifications within the spirit and scope of the invention will become apparent to those skilled in the art from this detailed description.